This invention relates to p-n tin oxide-cadmium telluride photovoltaic cells.
Cadmium telluride has long been identified as a strong candidate for low cost, thin film, photovoltaic applications because of its direct band gap and its ability to be doped n and p type permitting formation of a variety of junction structures, and to be deposited by a variety of techniques ranging from vacuum evaporation and chemical vapor deposition to electrodeposition and screen printing.
Photovoltaic cells using polycrystalline cadmium sulfide and cadmium telluride have been described in the past. For example U.S. Pat. No. 4,207,119 to Yuan-Sheng Tyan, which is hereby incorporated by reference, describes a cadmium sulfide/cadmium telluride solar cell in which oxygen atoms are present in the cadmium sulfide and/or cadmium telluride to improve the efficiency of the cell. A disadvantage of the examples shown in the Tyan patent is the use of low pressure conditions while depositing the cadmium telluride layers. It has also been found that the addition of oxygen must be kept low during the deposition process because the oxygen can result in substantial oxidation of the cadmium telluride source thereby suppressing the deposition process.
A problem in the past in fabricating photovoltaic cells containing cadmium telluride is the formation of low resistance electrical contacts to the cadmium telluride layer. One prior art technique is to chemically etch the cadmium telluride to form a telluride rich P+ conductivity region at the surface, then to deposit back metallization usually using high work function metals such as gold or nickel. However, contacts made by this method have not proven to be altogether satisfactory for several reasons including the sensitivity of the contact to excessive heat treatment causing chemical rections to form metal tellurides, and the present of surface oxides which tend to give rise to high contact resistances.
Therefore, it can be appreciated that a photovoltaic junction which can be formed at atmospheric pressure and under other conditions which lend themselves well to high throughput production, and a photovoltaic cell which has low resistance electrical contacts to the back conductor is highly desirable.